The present invention relates to antennas and, more specifically to multi-element antennas or array antennas.
In the past, efficient antennas have typically required structures with minimum dimensions on the order of a quarter wavelength of the radiating frequency. These dimensions allowed the antenna to be excited easily and to be operated at, or near a resonance frequency. This limited the energy dissipated in resistive losses and maximized the transmitted energy. Even at relatively high frequencies, these antennas tended to be large at the resonant wavelength. Further, as frequency decreased, the antenna dimensions had to increase in proportion.
In order to address the shortcomings of traditional antenna design and functionality, a new class of antennas was developed, known as meander line loaded antennas (MLAs). One such meander line loaded antenna is disclosed in U.S. Pat. No. 5,790,080 for MEANDER LINE LOADED ANTENNA, issued Aug. 4, 1998 to the present inventor, John T. Apostolos, and is hereby incorporated by reference.
An example of a meander line loaded antenna is shown in FIG. 1. The antenna consists of two vertical conductors 102 and a horizontal conductor 104. The vertical and horizontal conductors are separated by gaps 106. Also part of the antenna are meander lines 108, shown in FIG. 2, which inter-connect the vertical and horizontal conductors at the gaps 106. The meander lines 108 are designed to adjust the electrical length of the antenna. In addition, the meander slow wave structure permits switching of lengths of the meander line in or out of the circuit quickly and with negligible loss. This results in selectively changing the effective electrical length of the antenna. Such negligible-loss switching is possible because the meander line 108 includes alternating high and low impedance sections, with active switching devices located in the high impedance sections. This limits the current through the switching devices and results in very low dissipation losses in the switch. Accordingly, high antenna efficiency is maintained.
The meander line loaded antenna, as well as antennas in general, have had certain limitations when used in arrays. Currently, array antennas are very expensive because each antenna receives its own, separate signal. These signals, typically, are generated by using an external corporate feed network. These limitations are further magnified in the case of phased array antennas which achieve directional control by varying the phase of the transmission signal between different array elements, thus requiring phase control for each element. There is a need for an antenna with a fixed or directable pattern that has only a single feed point.
U.S. Pat. No. 5,943,011 entitled ANTENNA ARRAY USING SIMPLIFIED BEAM FORMING NETWORK discloses an example of an antenna array, or multi-element antenna and the feed network used for steering signals transmitted or received through the array. The signals coupled to and from each antenna element 51-58 are adjusted in phase by a network of r.f. hybrid devices 62.
U.S. Pat. No. 5,144,319 entitled PLANAR SUBSTRATE FERRITE/DIODE PHASE SHIFTER FOR PHASED ARRAY APPLICATIONS is an example of a phase shifter which can be used for an individual antenna element within an array. The patent shows the use of this shifter for each antenna element of a phased array.
U.S. Pat. No. 4,010,474 entitled TWO DIMENSIONAL ARRAY ANTENNA discloses a phase control network for the elements of a two dimensional array.
U.S. Pat. No. 5,949,303 entitled MOVABLE DIELECTRIC BODY FOR CONTROLLING PROPAGATION VELOCITY IN A FEED LINE discloses a single phase shifter for use with multiple array elements. As shown in FIG. 1, a feed conductor line 3 includes a source input 6 and multiple antenna element outputs T1-T4. A moveable dielectric material located between the feed line 3, or the carrier plate 5 thereof, and a ground plane 4, controls the propagation velocity of signals coupled through the feed line 3. In this manner a mechanical adjustment is made which determines the phasing of multiple antenna elements.
The prior art very clearly shows the level of complexity that is required for the use of multiple element antenna arrays, both in the need for individual connections as well as the requirements for phase control. They demonstrate the need for simplified coupling and phase control approaches.
It is, therefore, an object of the invention to provide an inexpensive, array antenna having only a single feed point.
It is an additional object of the invention to provide an array antenna having only a single feed point, which includes phase control for individual array elements.
In accordance with the present invention, there is provided an array antenna capable of use with a single signal feed and including a transmission line oriented substantially parallel to a ground plane, which transmission line has a multiplicity of sequential sections with each sequential section having a different spacing from the ground plane than each of its immediately adjacent sequential sections. Once interconnected, these sequential sections may be referred to as a stepped or varied impedance transmission line in which the sections spaced further from the ground plane become active antenna elements. In one embodiment, an electrical spacing between the transmission line and ground plane affects impedances of the transmission line. Varying this electrical spacing enables relative phase control between the active elements and phased-array directional control of the antenna.